In the manufacture of transparent containers such as glass bottles and jugs, various types of anomalies can occur in the sidewalls, heels, bottoms, shoulders and/or necks of the containers. These anomalies, termed "commercial variations" in the art, can affect commercial acceptability of the containers. It has heretofore been proposed to employ electro-optical inspection techniques for detecting commercial variations that affect the optical properties of the containers. The basic principle is that a light source is positioned to direct light energy onto the container, and a camera is positioned to receive an image of the portion of the container illuminated by the light source. The light source may be of uniform intensity, or may be configured to have an intensity that varies across one dimension of the light source. Commercial variations in the portion of the container illuminated by the light source are detected as a function of light intensity in the image of the illuminated container received and stored at the camera.
U.S. Pat. Nos. 4,378,493, 4,378,494, 4,378,495 and 4,601,395, all of which are assigned to the assignee of the present application, disclose inspection techniques in which glass containers are conveyed through a series of positions or stations where they are physically and optically inspected. At one optical inspection station, a glass container is held in vertical orientation and rotated about its central axis. An illumination source directs diffuse light energy through the container sidewall. A camera, which includes a plurality of light sensitive elements oriented in a linear array parallel to the vertical axis of the container, is positioned to receive light transmitted through a vertical strip of the container sidewall. The output of each element in the linear array is sampled at increments of container rotation, and event signals are generated when the magnitude of adjacent signals differ by more than a preselected threshold. An appropriate reject signal is produced and the container is sorted from the conveyor line.
A problem is encountered in the manufacture of glass containers from recycled glass in that materials having different thermal expansion characteristics can become mixed in a single container. For example, it has been found that clear cookware, having very low thermal expansion characteristics, can become mixed with the glass for recycling. Any unmelted particles of the cookware that appear in the container create stress points on cooling that can fracture or become sites for later failures. Other inhomogeneities that can appear in the glass and cause stress variations include stones or bits of refractory material from the glass forehearth or spout. It is thus necessary to provide a method and system for detecting stress and opaque non-stress variations in the containers. However, space is limited in the existing inspection systems, and the various inspection stations in the systems in place cannot readily accommodate additional inspection apparatus.
It has heretofore been proposed to employ crossed polarizers for detecting stress variations in the sidewalls of containers. Light energy directed through the crossed polarizers, and through a container positioned between the crossed polarizers, normally presents a dark field at the imaging camera in the absence of stress variations in the container sidewalls. However, a stress variation alters polarization of the light energy passing through the container sufficiently to present a bright spot at the camera against the otherwise dark background, indicative of the stress variation. See U.S. Pat. No. 4,026,656, assigned to the assignee hereof, which discusses such technology by way of background, and which proposes to employ infrared light energy and infrared polarization filters to reduce the background effects of ambient light.
It is a general object of the present invention to provide a method and apparatus for inspecting transparent glass articles, particularly glass containers, for commercial variations that affect optical characteristics of the containers. A more specific object of the present invention is to provide a method and apparatus of the described character that are particularly well suited for detecting both stress variations and opaque variations (stress and non-stress) in the container. Another object of the present invention is to provide a method and apparatus of the described character for detection of stress and opaque non-stress variations in containers at a single inspection station, using a single light source. A further object of the present invention is to provide a method and apparatus of the described character that are economical to implement and reliable over an extended operating lifetime. Yet another object of the present invention is to provide a method and apparatus of the described character that are adapted to be implemented at a single inspection station of an existing container inspection system.